PROJECT SUMMARY/ABSTRACT Systemic lupus erythematosus (SLE) is a multisystem autoimmune disease characterized by polyclonal B cell activation and the production of class-switched antinuclear antibodies (ANA). Transgenic mice overexpressing the key B cell cytokine, BAFF, develop an autoimmune disease resembling human SLE. Notably, BAFF binds to two alternative receptors expressed on transitional and mature B cells, BAFF-R and TACI. We recently published that deletion of TACI unexpectedly abrogates humoral autoimmunity in BAFF-Tg mice. Mechanistically, BAFF promotes the expansion of immature transitional B cells characterized by increased surface TACI expression. These TACIhi cells exhibit an activated, cycling phenotype and express high-levels of activation-induced cytidine deaminase (AID) required to mediate class-switch to pathogenic Ig isotypes. Consistent with a direct role for this subset in BAFF-mediated autoimmunity, TACIhi transitional cells are enriched for autoreactive BCR specificities and produce class-switched autoantibodies ex vivo. In this proposal we will test the hypotheses that TACIhi transitional cells are the predominant source of class-switched autoantibodies in BAFF-Tg mice; that coordinate signals via the BCR and TLR function to drive TACI expression in this subset; and that BCR engagement with self-ligand promotes TACI expression on human B cells in health and disease. We will test these hypotheses via the three Specific Aims: In Aim 1, we will determine the relative contribution of mature vs. transitional cells to BAFF-mediated autoimmunity. Using a novel ?fate-mapping? strategy, we will examine whether autoantibody-producing cells in BAFF-Tg mice are derived from transitional or mature B cell subsets. In parallel, we will test whether mature B cells are required for BAFF-Tg autoimmunity by deletion of BAFF receptor, a B cell surface receptor required for B cell survival beyond the transitional stage. In Aim 2, we will determine the B cell-intrinsic signals that promote surface TACI expression in vivo. This question will be addressed using a series of murine knock-out and transgenic crossed onto the BAFF-Tg background. In Aim 3, we will determine whether BCR engagement with self-ligand promotes TACI expression on human B cells in health and disease. We will achieve this by tracking intrinsically auto-reactive transitional B cells in: healthy control subjects; in a cohort of bone marrow transplant subjects where B cell reconstitution occurs within a high BAFF environment; and in human lupus patients stratified by serum BAFF levels.